Recommended Planescape Books

So I'm a new poster here, but I've been browsing this place (including the old website, I believe), the mimir website, Wandering Genasi Tavern, pathguy's planescape section, and a number of other websites for years. Despite this I have only managed to get one actual Planescape book, The Planar Handbook for 3.5

Anyway, I've been strongly considering DM'ing a Planescape game for 5e lately, but I feel like I've about exhausted most of the online resources I could find for ideas. Since I couldn't decide what to look further into, I figured I'd ask you guys, seeing as how this seems to be one of the premier places for Planescape knowledge. Are there any Planescape books (for any edition) that you think would be useful for running a game in 5e that cover things that aren't already done to death online? As well, do you guys know any of the more reclusive Planescape websites in various corners of the internet that I might have missed that would be useful? I'm especially interested in the inner planes (particularly the quasi, para, and similar areas) and planetouched, but other information is useful as well. Any suggestions are welcome.

At the moment, you're pretty much stuck with 2E resources for the best canonical information. 3E had a few books (Manual of the Planes, primarily), but didn't really develop the setting beyond a broad overview. 4E took everything and shook it up in a blender, so it's not quite as useful. And 5E is just too new to have much information out there yet.

If you can find a copy, The Planewalker's Handbook is basically a distillation of the entire setting into its choicest bits. Similarly, The Inner Planes book goes over each inner plane and fleshes them pretty decently (though some are better fleshed out than others). Both are 2E books though. Basically, you're stuck with out of print books (or pdfs, if you can find them) for the best information and setting voice.

Alternatively, you could start a thread and pick the collective brain trust of the board here. We each have our own take on the setting, and we're pretty good at tossing out interesting ideas.

In the Cage: A Guide to Sigil
The Factol's Manifesto
Uncaged: Faces of Sigil
A Player's Primer to the Outlands
City of Brass boxed set (from Necromancer Games)
The Inner Planes
Hellbound: The Blood War
A Guide to the Ethereal Plane
A Guide to the Astral Plane
Tales From the Infinite Staircase
Fiendish Codex I: Hordes of the Abyss
Fiendish Codex II: Tyrants of the Nine Hells
Vortex of Madness
Beyond Countless Doorways (Malhavoc Press)
Dark Roads & Golden Hells (Open Design)
Secrets of the Lamp (Al-Qadim)
The Plane Below: Secrets of the Elemental Chaos
The Shadowfell: Gloomwrought and Beyond
Classic Play: Book of the Planes (Mongoose)
The Manual of the Planes (3rd edition)
The Manual of the Planes (1st edition)
Seven Civilizations (Penumbra)